Lithium ion secondary batteries, which are storage battery with high energy density, are used as major power sources of various portable devices. In recent years, electrode structures or current collectors have been devised to increase the power of lithium ion secondary batteries. Lithium ion secondary batteries are expected to be developed as power sources for hybrid electric vehicles (HEV) by utilizing its compact and light weight features. Such a lithium ion secondary battery includes an electrode group in which strip shaped positive and negative electrode plates, each including a mixture layer and a current collector, and a separator providing electrical insulation between the electrode plates and holding an electrolyte therein are spirally wound. As the separator, a microporous thin sheet mainly containing polyethylene having a thickness of several tens μm is used.
To increase the power of a lithium ion secondary battery, it is necessary to reduce a member resistance and a reaction resistance. To reduce a member resistance and a reaction resistance, for example, besides forming leads with a large thickness and optimizing welding conditions, the following method can be used. After providing an exposed portion in which the mixture layer does not exist is provided at one end of the current collector along a long side direction in each of the electrode plates, the electrode plates are arranged so that an exposed portion of the positive electrode current collector is located at one end of the electrode group and an exposed portion of the negative electrode current collector is located at the other end of the electrode group. Then, the exposed portions of the current collectors are assembled and welded to ensure uniform channels for electrons in the strip shaped electrodes.
To reduce a reaction resistance, besides increasing the ratio of the area of an active material with respect to the area of the mixture layer and optimizing the amount of a conductive material, a method can be used in which the area of the positive and negative electrode plates is increased to reduce a current density, thereby suppressing a voltage drop when a discharge reaction occurs. In fact, an electrode plate of a lithium ion secondary battery for high power application, which is currently under development, is formed to have an area substantially equal to or larger than the double of the area of an electrode plate of a lithium ion secondary battery for various portable devices when the two electrode plates are compared in the same capacity.
When an internal short-circuit occurs in a lithium ion secondary battery developed specifically for high power application, its high power property is increased and a short-circuit current is accordingly increased. Specifically, in a lithium ion secondary battery for high power application, a reaction resistance Rr is reduced for the purpose of achieving high power output. Thus, the reaction resistance Rr provided for rate-determining of a short-circuit current I is small and a short-circuit current (I=V/Rr where V is a standardized voltage) is large. As described above, in a lithium ion secondary battery for high power application, a short-circuit current which flows in the battery when an internal short-circuit occurs is relatively large. Accordingly, the temperature inside of the lithium ion secondary battery is rapidly increased due to Joule heat and there is a possibility that fume emission occurs in the battery.
In general, to ensure the safety of a lithium ion secondary battery, a test in which an electrically abnormal state such as an overcharge, an overdischarge and the like is simulated and a test in which an external physical impact such as sticking of a nail therein or crush by application of pressure is simulated are conducted. Moreover, a safety mechanism for preventing explosion, combustion and fume emission of the battery is adopted.
Specifically, for example, in a HEV pack battery in which several tens cells of lithium ion secondary batteries are connected in series, a safety mechanism in which a charge/discharge current is forcedly stopped by a battery control system is established for an electrically abnormal state such as an overcharge, an overdischarge and the like. Since the battery control system can not cover a damage caused by an external physical impact such as sticking of a nail therein and the like, a safety mechanism such as a mechanism for accommodating the pack battery in a strong exterior case which can withstand an external physical impact and the like has to be established for an external physical impact.
For example, Patent Reference 1 discloses that when a high capacity, high power and long life lithium ion secondary battery in which a defective such as a short-circuit and the like does not occur between positive and negative electrodes even after a lapse of time can be achieved by using a negative electrode current collector of which, when a negative electrode plate is pressed so that a bulk density of a negative electrode mixture is set to be a predetermined level, an area increase rate per unit area becomes 0.5% or more and 2% or less. Patent Reference 1 further discloses that to obtain such high capacity, high power and long life lithium ion secondary battery, a copper foil having a thickness of 9 μm or more and a surface roughness (Ra) of 0.10 or more is preferably used as the negative electrode current collector.    Patent Reference 1: Japanese laid-Open Publication No. 2001-210330